Currently, as bonding wires for a semiconductor device (hereinafter referred to as bonding wires) for bonding electrodes on semiconductor elements with wiring such as external leads, fine wires approximately 15 to 50 μm in wire diameter are mainly used. As a method for bonding the bonding wire, a thermosonic bonding process is used commonly together with a general-purpose bonding machine, a capillary jig adapted to connect a bonding wire passed therethrough, and the like. A bonding wire bonding process involves heat-melting a wire tip by arc heat input, forming a ball by surface tension, pressure-bonding (hereinafter referred to as ball bonding) the ball to an electrode of a semiconductor element heated in a range of 150 to 300° C., then forming a loop, and pressure-bonding (hereinafter referred to as wedge bonding) a wire portion to an electrode on the side of the external lead to complete the process. An electrode structure made up of an alloy film composed principally of Al and formed on an Si substrate or an electrode structure in which the electrode on the side of the external lead is plated with Ag or Pd is often used for the electrode on the semiconductor element to which the bonding wire is bonded.
Excellent ball forming property, ball bondability, wedge bondability, loop forming property, and the like are required of the bonding wire. As a bonding wire material which generally satisfies these performance requirements, Au is used mainly. In late years, against the background of skyrocketing Au prices, intensive development of bonding wires have been going on using comparatively inexpensive materials as substitutes for Au. Development examples include a bonding wire having a structure in which a surface of Cu is coated with Pd. This bonding wire features generally improved performance achieved mainly by inhibiting oxidation of the Cu, and is used in a cutting-edge area of LSI (Large Scale Integration).
In the future development of bonding wires, there is strong demand to accommodate high-density packaging resulting from further performance improvement and downsizing of semiconductor devices. In the high-density packaging, to reduce signal delays among LSI layers, fragile low-k materials are sometimes used as interlayer insulating materials, often posing a problem of damage to semiconductor elements. Because spacing among adjacent electrodes is small, making it necessary to reduce the wire diameter of the bonding wire, high wedge bondability is required of the bonding wire. In order to secure electric conductivity with small wire diameter, it is desirable that specific resistance of the material used for the bonding wire is low. As a bonding wire material in such a high-density area, Au is often used because of softness, high wedge bondability, and comparatively low specific resistance.
Attempts are made to use Ag as a bonding wire material in order to solve the above problem with high-density packaging and provide a bonding wire material less expensive than Au. Ag, whose Young's modulus (about 83×109 N/m2) is approximately equal to the Young's modulus of Au (approximately 80×109 N/m2), and lower than the Young's modulus of Cu (approximately 130×109 N/m2), is expected to cause less damage in ball bonding to fragile semiconductor elements and provide good wedge bondability. The specific resistance of Ag (1.6 μΩ·cm) at around room temperature is lower than the specific resistance of Cu (1.7 μΩ·cm) and specific resistance of Au (2.2 μΩ·cm), and thus it is considered that Ag is suitable as a bonding wire material in high-density packaging from the viewpoint of electric characteristics as well.
However, the bonding wire made of Ag (hereinafter referred to as an Ag bonding wire) has a problem of low bonding reliability and low loop stability in high-density packaging. The bonding reliability evaluation is carried out for the purpose of evaluating the life of bonds in an operating environment of the actual semiconductor device. Generally, high temperature storage testing and high temperature high humidity testing are used for bonding reliability evaluation. The Au bonding wire has a problem of being inferior to a bonding wire made of Au (hereinafter referred to as an Au bonding wire) in the life of ball bonds in high temperature high humidity testing. With high-density packaging, small ball bonding is done, decreasing the area which contributes to bonding, and thus it is increasingly difficult to ensure a long life time.
Regarding the loop stability, a failure called a spring failure poses a problem. The spring failure is a phenomenon in which the loop of bonding wires bends in a bonding step, contacting each other and causing a short circuit. With high-density packaging, because spacing among adjacent bonding wires is small, it is strongly desired to prevent spring failures. Spring failures, which occur in proportion to decreasing wire strength, often poses a problem in high-density packaging, which involves reduced wire diameter.
As a method for solving these problems, a technique is disclosed for creating an alloy by adding various elements to Ag, but balls become harder with increases in the concentration of alloy elements, causing chip damage during ball bonding. These problems are responsible for hindering the spread of Ag bonding wires.
Regarding the development of Ag bonding wires for the purpose of improving bonding reliability, for example, Patent Literature 1 discloses a bonding wire containing one or more of Pt, Pd, Cu, Ru, Os, Rh, and Ir for a total of 0.1 to 10 wt %, where Pt is contained 10 wt % or less, Pd is contained 10 wt % or less, Cu is contained 5 wt % or less, Ru is contained 1 wt % or less, Os is contained 1 wt % or less, Rh is contained 1 wt % or less, and Ir is contained 1 wt % or less, with the balance being attributable to Ag and incidental impurities.
For example, Patent Literature 2 discloses an Ag—Au—Pd ternary alloy bonding wire for semiconductor elements made up of Ag with a purity of 99.99 mass % or above, Au with a purity of 99.999 mass % or above, and Pd with a purity of 99.99 mass % or above, wherein Au is contained 4 to 10 mass %, Pd is contained 2 to 5 mass %, and additional oxidizing non-noble metal elements are contained 15 to 70 mass ppm and the balance is made up of Ag; and the bonding wire is subjected to a thermal annealing process before continuous die drawing, subjected to thermal refining treatment after the continuous die drawing, and used for ball bonding in a nitrogen atmosphere.
Regarding the development of Ag bonding wires for the purpose of improving loop stability, a technique is disclosed for controlling tensile strength and 0.2% proof stress by thermomechanical treatment. For example, Patent Literature 3 discloses a bonding wire whose tensile strength measured in a 523K temperature atmosphere after being heated in the temperature atmosphere for 15 to 25 seconds is higher than 0.2% proof stress measured in a 298K temperature atmosphere.